1. Field of Invention
The present invention relates to an electronic ballast. More particularly, the present invention relates to an electronic ballast capable of extending life of the fluorescent lamp.
2. Description of Related Art
A fluorescent lamp is an evacuated glass tube with a small amount of mercury in the tube. The tube is lined with an adherent layer of a mixture of phosphors. Some of the mercury vaporized at low pressure within the tube and a filament or cathode in each end of the tube is heated to emit electrons into the tube, ionizing the gas. A high voltage between the filaments causes the mercury ions to conduct current, producing a glow discharge that emits ultraviolet light. The ultraviolet light is absorbed by the phosphors and re-emitted as visible light.
In general, the fluorescent lamp is driven by an electronic ballast. Referring to FIG. 1, a circuit diagram schematically showing an electronic ballast in the prior art. The circuit of electronic ballast in the prior art includes a main circuit and a preheating circuit. It is required that a preheating is applied to vaporize the mercury inside the lamp before the ignition of the fluorescent lamp. Referring to FIG. 1 again, the secondary side of the transformer L3 includes a winding L3D, a winding L3E, and a winding L3F. When the primary side of the transformer L3 starts to receive an alternative current, windings L3D, L3E and L3F induce a voltage and a current simultaneously. And the three windings are provided for applying a preheating on the filaments of the fluorescent lamp.
FIG. 1 is a circuit diagram schematically showing an electronic ballast used to drive the fluorescent lamp in the prior art. However, this electronic ballast in the prior art suffers one problem as described below. Referring to FIG. 1, when the electronic ballast drives the lamp after completion of the preheating through windings L3D, L3E, and L3F, there is still a current, i.e. a glow current, flowing through the filaments of the fluorescent lamp. This glow current continuously heats the filaments of the lamp, and therefore leads to damage of the lamp life.
To solve the previously-mentioned flaw of electronic ballast for driving the fluorescent lamp in the prior art of FIG. 1, one circuitry of the lamp (published in the patent CN. 1400852A) was proposed. As shown, FIG. 2 is a circuit diagram schematically showing another electronic ballast in the prior art. Referring to FIG. 2, a transformer T2 is used for preheating the lamp. When the circuit activates, an alternative current received by the primary side T22 of the transformer T2 is rectified through a diode D1. Also, the rectified voltage is divided through resistors R1 and R2 and outputted to a capacitor C2. The capacitor C2 and resistors R1 and R2 are specifically designed for providing a preheating period of time required for the lamp. When the capacitor C2 is charged to a predetermined voltage value, i.e. completion of the preheating, the SCR is triggered so that the transformer T2 is turned off. Therefore, due to the cut-off state of the transformer T2, no glow current flowing through filaments of the lamp appears. By doing so, flaws of the electronic ballast in FIG. 1 can be resolved.
Yet, there is still a flaw of the electronic ballast mentioned above. Usually, a self-excitement resonant circuit is designed to drive the transistors Q1 and Q2 in the circuit for the purpose of cost-effective consideration. But with usage of the transformer T2 it is quite difficult and complicated for the designing of the circuit to drive the transistors Q1 and Q2 that could meet the requirements for the lamp during period of the preheating and the steady state subsequently.